Inflammatory responses to the microbiota are proposed as contributory to the initiation and/or progression of human colorectal cancer (CRC). However, molecular links between members of the microbiota and colon carcinogenesis are poorly defined. The human colon anaerobic bacterium, enterotoxigenic Bacteroides fragilis (ETBF), is a molecular subtype of B. fragilis that produces a potent metalloprotease toxin called the B. fragilis toxin (BFT). ETBF is a potent inducer of colon tumorigenesis in a murine model through induction of a selective procarcinogenic intracolonic Th17 immune response. ETBF colon tumorigenesis requires BFT expression. BFT activates several procarcinogenic signaling pathways in colonic epithelial cells (CEC) by binding to a specific receptor localized to CECs. However, the BFT CEC receptor and its relationship to BFT-induced oncogenic signaling, colitis and colon tumorigenesis remain to be elucidated. Through subtraction array and shRNA strategies, GPR35, a G protein-coupled receptor (GPCR), is now identified as the putative BFT receptor. This proposal will test the hypothesis that GPR35, known to be highly expressed in the colon and previously linked to gastric cancer induction, is the CEC receptor hijacked by BFT and a critical contributor to colon carcinogenesis, thereby, providing a direct molecular link between a common microbiota member and CRC pathogenesis. We will test this hypothesis and further define how GPR35 contributes to BFT-induced CEC oncogenic signaling using knockout and knock-in CEC strategies plus GPR35 and ?-arrestin KO murine models. GPRCs represent the largest family of cell surface receptors within the human genome and powerful targets for new drug discovery. Thus, establishing a role for GPR35 or its isoforms in colon carcinogenesis may advance the prevention and/or therapy of human CRC.